Wireless Communications

Transcription

1 2. Physical Layer DIN/CTC/UEM 2018

2 Periodic Signal Periodic signal: repeats itself in time, that is g(t) = g(t + T ) in which T (given in seconds [s]) is the period of the signal g(t) The number of cycles (or periods) per second is the frequency f of the signal g(t) (given in Hertz [Hz]) f = 1 T Amplitude is the signal strength Intantaneous amplitude: amplitude at some time t Maximum and minimum amplitudes: maximum and minimum values reached by the signal

3 Analog Signal Can assume any real numeric value In general, it is continuous in time, that is, it is defined for any time t Maintains direct relationship with expressed magnitude Example: electric voltage of a microphone is proportional to the pressure caused by the sound/voice

4 Digital Signal Can assume only a finite set of values In general, it is discrete in time, that is, it is only defined for some instants of time t i, i = 0, 1,... In general, digital signal is obtained from an analog source Analog-to-Digital Converter (ADC) Digital-to-Analog Converter (DAC)

5 Sampling and Digitalization Technique for analog-digital conversion

6 Digitalization Example

7 Bit Rate and Baud Rate Bit rate Every second produces N samples Each sample uses B bits Therefore: bit rate = N B Baud rate Each symbol carries b bits Therefore: baud rate = N B b

8 Information in the Digital Format Advantages of manipulation in the digital format: Economy: advances in digital circuits Security: easier to encript Advantages of transmission in the digital format: Data integrity: increased noise immunity Channel usage: easier to multiplex multiple sources

9 Time and Frequency Domains Time domain: independent variable is the time Frequency domain: independent variables are frequency and phase Example: sine wave (fundamental wave) time domain: s(t) = A sin(2πft) s(t) R frequency domain: S(f ) = A δ(f ) S(f ) C δ(.) is the impulse function

10 Decomposition of Periodic Waves Any periodic signal of frequency f can be expressed as a summation (often infinite) of sine waves Component sine waves have frequencies that are multiple of the frequency of the orginal signal and are known as harmonics: sine wave with frequency f is the first harmonic sine wave frequency 2 f is the second harmonic and so on Each sine wave represents a component (an impulse) in the frequency domain time domain: frequency domain:

11 Decomposition of Periodic Waves time domain: frequency domain: The more harmonics are added to the summation, the more the resulting signal resembles the original signal If the mean value of the periodic wave is nonzero, this value is represented with a zero frequency component (f = 0)

12 Spectrum Non-Periodic Signals When the signal is non-periodic, the spectrum ceases to be formed by discrete frequencies and becomes continuous The bandwidth of the analog signal is delimited by the two frequencies (f 1 and f 2 in the figure below) where the signal power is half the maximum power

13 The Multiplexing Problem How to divide the channel among all users?

14 FDM Frequency Division Multiplex

15 TDM Time Division Multiplex

16 Combination of FDM and TDM

17 CDM - Direct Sequence Code Division Multiplex, Direct Sequence

18 Example CDM - Direct Sequence 4 users (A, B, C and D), each with his/her own spreading code Codes are orthogonal (i.e., A B = A C =... = C D = 0) Sending of information uses the spreading code For example: User C transmits code C to send bit 1 User C transmits code C to send bit 0 In the channel occurs the sum of the transmitted signals (S 1 to S 6 in the figure) For decoding, the receiver must multiply the received signal and the desired spreading code (in the figure, the C code)

19 Example CDM - Direct Sequence

20 CDM - Frequency Hopping Code Division Multiplex, Frequency Hopping

21 Conversion of Bits into Signals Source of information produces bits In general bits are not good for data transmission It is necessary to convert bits into a better signal, appropriate to the channel Objectives: Reduce the bandwidth of the encoded signal Concentrate the encoded signal spectrum in the channel bandwidth, thus reducing attenuation and distortion Eliminate high-frequency components, thus reducing interference between physically close channels Eliminate low frequency components of the encoded signal Enable AC coupling, which ensures better insulation and the possibility of remote power supplying (repeaters, terminals) with transformers Shape the spectrum of the encoded signal in a way to facilitate the extraction of a clock signal for the synchronizing function

22 Conversion of Bits into Signals Conversion of bits into digital signals Digital encoder and decoder Conversion of bits into analog signals Modem (modulator e demodulator)

23 Unipolar Coding Uses presence/absence of polarity; for example: Bit 0: absence of polarity Bit 1: presence of polarity Easy implementation Reasonable spectral efficiency Does not allow AC coupling (signal contains DC component and low frequency) Does not allow self-synchronism (difficult to extract clock from signal)

24 Bipolar Coding It uses two different polarities Non-return-to-zero (NRZ) Polarity remains the same during the period of a bit Examples: NRZ-Level (NRZ-L): signal follows change of bit NRZ-Inverted (NRZ-I): signal changes if next bit is 1 Return-to-zero (RZ) Bits represented by pulses that occupy only part (50% in general) of the period of a bit Signal remains at zero for the remainder of the bit time

25 NRZ Encoding Advantages and disadvantages similar to unipolar coding However, it allows AC coupling

26 RZ Encoding Worse spectral efficiency than NRZ However, it allows easy synchronization

27 Alternate Mark Inversion (AMI) Encoding Bipolar coding: Bit 0: no polarity Bit 1: alternate polarities Immunity to inversions of polarities Absence of low frequency components Good spectral efficiency Used by the American T1 (1.544 Mbps)

28 Manchester Encoding Manchester: transition in the middle of each bit Bit 0: downward transition Bit 1: upward transition Used by IEEE (Ethernet) Manchester Differential Bit 0: transition at the beginning of the bit Bit 1: no transition at the beginning of the bit Used by IEEE (Token Ring) Advantages Easy implementation Easy synchronization No CC component Disadvantages The baud rate is twice the bit rate Requires greater bandwidth than NRZ

29 Manchester Encoding

30 Other Coding Schemes Bipolar with 8 Zeros Substitution (B8ZS) Based on AMI Sequency of is replaced by if previous pulse is positive if previous pulse is negative High Density Bipolar 3 Zeros (HDB3) Based on AMI Forth zero in the sequency 0000 is always transmitted with a pulse that violates the rule of alternation First zero in the sequency 0000 may be transmitted with a pulse in accordance to the rule of alternation Used by the European E1 (2.048 Mbps)

31 Power Spectrum

32 Digital Modulation Carrier: y(t) = A cos(2πft + φ) in which A is the amplitude, f is the frequency and φ is the phase Frequency Shift Keying (FSK) Modulate (change) the frequency f Amplitude A and phase φ remain unchanged Amplitude Shift Keying (ASK) Modulate (change) the amplitude A Frequency f and phase φ remain unchanged Phase Shift Keying (PSK) Modulate (change) the phase φ Amplitude A and frequency f remain unchanged

33 Digital Modulation digital signal ASK FSK PSK

34 Digital Modulation ASK Encode bits wiuth different carrier amplitudes Inefficient and not immune to noise Up to 1200 bps on phone lines; most popular with fiber optics FSK Encodes bits with different carrier frequencies More robust than ASK PSK Encodes bits with different carrier phases Binary PSK (BPSK) uses 2 phases (0 o e 180 o, for example) to represent bits 0 and 1 Quaternary PSK (QPSK) uses 4 phases (0 o, 90 o, 180 o e 270 o, for example) to represent the pairs of bits 00, 01, 10 and 11

35 Spectral Density ASK and PSK f c = carrier frequency

36 Quadrature Amplitude Modulation (QAM) Generalization of PSK that combines phase and amplitude modulations Encodes bits with different pairs of amplitude/phase of the carrier For example, Modem V.29 (9600 bps) uses 16-QAM with eight phases and two amplitudes totaling 16 symbols Each symbol carries 4 bits and therefore the symbol rate (baud rate) is 2400 baud Allows higher spectral efficiency because it can pack more bits by symbol.

37 PSK/QAM Constellation Symbols in PSK and QAM modulations are represented by pairs (amplitude, phase), as if they were complex numbers in polar form Graphically, symbols are represented by points in the complex plane (a) QPSK (ou 4-PSK) (b) 16-QAM (12 fases e 3 amplitudes) (c) 64-QAM

38 Electromagnetic Spectrum

39 ISM band in the USA Industrial, Scientific, Medical MHz: transmitter up to 1 W microwave oven up to 750 W industrial heater up to 100 kw radar up to 1000 kw GHz: transmitter up to 1 W microwave oven up to 900 W GHz transmitter up to 1 W

40 ISM Band in Brazil MHz (30 khz band) MHz (4 khz band) MHz (326 khz band) MHz (40 khz band) MHz (26 MHz band) GHz (100 MHz band) GHz (250 MHz band) GHz (500 MHz band) GHz (1 GHz band) GHz (2 GHz band)

41 Communication Satellites Used for Telephone Television Disadvantages: Orbit congestion (GEO satellites) Each GEO satellite occupies 2 degrees of the circumference High cost, high risk, high delay, low privacy Limited life (fuel)

42 Communication Satellites Satélites GEO (Geostationary), MEO (Medium Earth Orbit) and LEO (Low Earth Orbit) Sats needed is the number of satellites for planetary coverage

43 Communication Satellites Bands used

44 Iridium 66 LEO satellites, each with up to 48 cells Transmission between satellites Voice and data up to 2.4 kbps and Internet up to 10 kbps Company still in business (check (a) Satellites (b) Cells

45 Mobile Phone System 1st Generation (1G), 1985 analog voice AMPS, TACS, NMT, etc. 2nd Generation (2G), 1992 digital voice D-AMPS, GSM, CDMA (IS-95), etc. 2nd Generation Transitional (2.5G, 2.75G) packet switching and data communication GPRS/EDGE, CD2000-1x, etc. 3rd Generation (3G), 2003 digital voice + data UMTS, CDMA2000, etc. 4th Generation (4G), 2013 digital voice + multimedia, IP network LTE, WiFi, etc.

46 AMPS Advanced Mobile Phone System Area of coverage divided into cells Allow frequency reuse Transmission using lower power

47 AMPS Some components Mobile station (MS) or mobile terminal Base Station (BS) Mobile Switching Center (MSC) A little bit of jargon Camping: MS camps in a cell Handoff or handover: MS changes cell during call Paging: sent by BS to MS for call search and alert Cell Info: broadcasted by BS, informs cell identification and characteristics Uses TDM-FDM on radio link with channel bandwidth of 30 khz 832 channels in the 800 MHz band, plus channels added in the MHz band

48 AMPS Cell Selection (initial camping) When turned on, MS searches for a cell to camp, usually the strongest of a list of preferred carriers Cell Reselection (cell change with no call in progress) Continuously MS checks the intensity of all the cells it can receive A report with this list is sent to BS The cell change occurs when a cell becomes better than the current one Handoff (cell change during a call) MS sends to BS values about call quality in progress and the strengths of neighboring cells System decides when handoff should occur and informs MS

49 Digital AMPS (D-AMPS) North American system of 2nd generation, specified by IS-136 Co-existence with the analog AMPS standard (maintains same channel structure in the radio interface) Each analog channel divided into 3 digital channels Compatibility requirement with AMPS impaired system

50 GSM Global System for Mobile Communication, European system of 2nd generation Uses TDM-FDM in radio link with 200 khz channels System with the largest number of subscribers worldwide Improved for data communication (the generation 2.5G) General Packet Radio Service (GPRS): up to 60 kbps Enhanced Data Rates for GSM Evolution (EDGE): up to 380 kbps

51 GSM GSM Structure: Combination of TDM and FDM with TDM frame with 8 slots and separate channels for uplink and downlink

52 CDMA (IS-95) North American 2nd generation system specified by IS-95 Uses CDM instead of TDM or FDM, with channel bandwidth of 1.25 MHz

53 Third/Forth Generations 3G Voice, messaging, multimedia and Internet access Rates up to 2 Mbps System with global access UMTS Uses W-CDMA with channel bandwidth up to 20 MHz Allow handoff with GSM CDMA-2000 Uses W-CDMA with channel bandwidth up to 20 MHz Based on IS-95 4G Mobile broadband Internet access, high-definition mobile TV, video conferencing, etc. Rates up to 100 Mbps (peak rate in mobile environment) and 1 Gbps (peak rate in indoor environment) All IP network Uses CDMA or OFDM with channel bandwidth up to 20 MHz